The present invention relates to the manufacture of copper microalloys, particularly the casting copper by conventional batch casting, semi-continuous casting or continuous casting and of rolling tough-pitch copper or microalloyed copper. It provides the addition of lead or refining to a final concentration of lead equal to or higher than 200 ppm. This allows the casting of copper microalloyed with elements such as S, Se, As, Sb, Bi, Sn, Zn, Ni, Fe, Ag and Te in amounts of the order of tens of weight ppm.
This invention also relates to a pre-heating treatment which has been discovered to be necessary to let some copper microalloys with a lead concentration equal to or higher than 200 ppm have the same strain strength, annealing temperature, half-softening temperature and recrystallization temperature as those obtained for tough-pitch copper, and an electrical conductivity equal to or higher than 101.5% IACS.
Until recently, it was accepted that a lead content in melt copper higher than 15-20 weight ppm, and a high content of other impurities was undesirable due to reduction of the electrical conductivity and the formation of high number of defects and bubbles in a phenomena known as hot-shortness. This meant that only tough-pitch copper could be cast. Thus blister copper or copper scrap refined by pyrometallurgical methods, which gave a lead content lower than 15-20 weight ppm, and decreased sufficiently impurities to produce high electrical conductivity copper, was not technologically competitive compared to electrolytically-refined copper.
Despite all the related handicaps in the fire-refining process and in the products, some companies developed different slagging agents in order to achieve the purity of tough-pitch copper while avoiding the expensive process of electrolytical refining. Nevertheless, it was difficult to decrease lead content to values lower than 15-20 weight ppm by fire-refining. The fire-refined copper produced was a high quality product, with electrical, thermal and mechanical properties very similar to electrolytically-refined copper, but because of its high lead content, it was often impossible to cast or roll, or else the final product was brittle and susceptible to breakage due to the porosity in the metal.
It is also known that a pre-heating treatment at 550-650xc2x0 C. for one hour or longer before high cold-working (80% or more) on microalloys with a lead content higher than 15-20 ppm significantly decreases their annealing temperature, half-softening temperature and recrystallization temperature, and also increases the electrical conductivity. Some compositions of microalloyed copper, treated with this pre-heating process achieve similar mechanical, thermal and electrical properties to tough-pitch copper.
To find a solution to the above mentioned drawbacks, the inventors have carried out investigations which have led to this invention, providing in a process for the discontinuous, semi-continuous or continuous casting of copper or microalloyed copper, the addition of lead or refining to a final concentration of lead equal to or higher than 200 weight ppm. Surprisingly-enough, this allows the casting and rolling of microalloys with impurities such as S, Se, As, Sb, Bi, Sn, Zn, Ni, Fe, Ag and Te in concentrations of the order of tens of weight ppm.
The invention refers well to an optional pre-heating treatment at 550-650xc2x0 C. for 5-600 seconds which, when applied to some copper microalloys with a lead content equal to or higher than 200 weight ppm, leads to a decrease in their strain strengths, giving annealing temperatures, half-softening temperatures or recrystallization temperatures equal to or lower than 200xc2x0 C., thereby obtaining mechanical, thermal and electric properties similar to ETP-CU.
This invention is based on the following:
a) Lead concentrations higher than 200 weight ppm in copper and copper microalloys secure their castability by conventional casting (by batch, semi-continuous or continuous casting) and their rolling because of their low hot-shortness, and the number of breaks in the cast bar decreases. The improvement in the microstructure in terms of small number of voids and bubbles also ensures a small number of breaks at lower values of tensile strength and elongation than the statistically established.
b) Lead concentrations higher than 200 weight ppm secure the casting and rolling of copper microalloys containing microalloying elements such as S, Se, As, Sb, Bi, Sn, Zn, Ni, Fe, Ag and Te in the order of tens of weight ppm.
c) A pre-heating treatment at 550-650xc2x0 C. for 5-600 s on copper with impurity contents lower than 80 weight ppm of elements Sn, Zn, Ni, Ag, Cd, Sb, S and Fe that have been cast by the addition of lead or refining until a lead content in the solid product higher than 300 weight ppm decreases their half-softening temperature, annealing temperature and recrystallization temperature to values lower than 200xc2x0 C.